Interlexeme-space may occur on either side of any lexeme, but not within a lexeme.

Identifiers, ., numbers, characters, and booleans, must be terminated by a delimiter or by the end of the input.

((UNFINISHED))

Line endings are significant in Scheme in single–line comments and within string literals. In Scheme source code, any of the line endings in line-ending marks the end of a line. Moreover, the two–character line endings carriage-return linefeed and carriage-return next-line each count as a single line ending.

In a string literal, a line-ending not preceded by a \ stands for a linefeed character, which is the standard line–ending character of Scheme.

As a special case the characters #!/ are treated as starting a comment, but only at the beginning of file. These characters are used on Unix systems as an Shebang interpreter directive. The Kawa reader skips the entire line. If the last non-whitespace character is \ (backslash) then the following line is also skipped, and so on.

Whitespace characters are spaces, linefeeds, carriage returns, character tabulations, form feeds, line tabulations, and any other character whose category is Zs, Zl, or Zp. Whitespace is used for improved readability and as necessary to separate lexemes from each other. Whitespace may occur between any two lexemes, but not within a lexeme. Whitespace may also occur inside a string, where it is significant.

The lexical syntax includes several comment forms. In all cases, comments are invisible to Scheme, except that they act as delimiters, so, for example, a comment cannot appear in the middle of an identifier or representation of a number object.

A semicolon (;) indicates the start of a line comment. The comment continues to the end of the line on which the semicolon appears.

Another way to indicate a comment is to prefix a datum with #;, possibly with interlexeme-space before the datum. The comment consists of the comment prefix #; and the datum together. This notation is useful for “commenting out” sections of code.

Block comments may be indicated with properly nested #| and |# pairs.

#|
   The FACT procedure computes the factorial of a
   non-negative integer.
|#
(define fact
  (lambda (n)
    ;; base case
    (if (= n 0)
        #;(= n 1)
        1       ; identity of *
        (* n (fact (- n 1))))))

Most identifiers allowed by other programming languages are also acceptable to Scheme. In general, a sequence of letters, digits, and “extended alphabetic characters” is an identifier when it begins with a character that cannot begin a representation of a number object. In addition, +, -, and ... are identifiers, as is a sequence of letters, digits, and extended alphabetic characters that begins with the two–character sequence ->. Here are some examples of identifiers:

lambda         q                soup
list->vector   +                V17a
<=             a34kTMNs         ->-
the-word-recursion-has-many-meanings

Extended alphabetic characters may be used within identifiers as if they were letters. The following are extended alphabetic characters:

! $ % & * + - . / < = > ? @ ^ _ ~

Moreover, all characters whose Unicode scalar values are greater than 127 and whose Unicode category is Lu, Ll, Lt, Lm, Lo, Mn, Mc, Me, Nd, Nl, No, Pd, Pc, Po, Sc, Sm, Sk, So, or Co can be used within identifiers. In addition, any character can be used within an identifier when specified using an escape-sequence. For example, the identifier H\x65;llo is the same as the identifier Hello.

Kawa supports two additional non-R6RS ways of making identifiers using special characters, both taken from Common Lisp: Any character (except x) following a backslash is treated as if it were a letter; as is any character between a pair of vertical bars.

Identifiers have two uses within Scheme programs:

In contrast with older versions of Scheme, the syntax distinguishes between upper and lower case in identifiers and in characters specified via their names, but not in numbers, nor in inline hex escapes used in the syntax of identifiers, characters, or strings. The following directives give explicit control over case folding.

Syntax: #!fold-case

Syntax: #!no-fold-case

Note that colon : is treated specially for colon notation in Kawa Scheme, though it is a special-initial in standard Scheme (R6RS).

((INCOMPLETE))

The letter used for the exponent in a floating-point literal determines its type: